Sex-Linked Inheritance Problem Set

Problem 1: Crossing a white-eyed female and red-eyed male fly

In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will have white eyes? (White eyes are X-linked, recessive.)

Tutorial

Genotypes and phenotypes of parents

The female parent must be homozygous because she has the recessive white-eyed phonotype.

The male parent is hemizygous, red-eyed.

Genotypes and phenotypes of offspring

All of the females eggs will contain an X chromosome with the white-eye mutation.

The sperm will contain either a normal X chromosome or a Y chromosome.

Summary

We use a Punnett Square to predict the outcome of this cross Female offspring receive an X chromosome from both the sperm and egg. All females receive the dominant, red-eyed allele from their fathers and the recessive, white-eyed allele from their mothers.

(If necessary see the tutorial for problem #1 of the monohybrid cross problem set for a review of how to set up a Punnett Square.)